I have a pregnancy testing kit in my bathroom and the instructions say "Two lines means you are probably pregnant (however, one line may be lighter than the other, they do not have to match)".
Therefore, your test is detecting pregnancy hormone in your urine, so you ARE pregnant. However, it is possible the line is light because there is not enough pregnancy hormone in your body yet because it is too early. Wait 3 days and do the test again and see if the line comes up faint again.
I too was puzzled by a faint line recently, on several pregnancy tests, even a weeks after my missed period. I say my GP and got a positive result but then miscarried at 8 weeks.
I asked a midwife about this and she told me that a faint line could mean that it is either too early to test or might indicate that the pregnancy is not established and therefore not producing enough hCG.
Best to speak to your GP. Good luck!